Re: started my cnc conversion for Precision Matthews PM-25MV Mill

Originally Posted by 109jb

That link is for calculating the force produced by an ACME screw. The OP has ballscrews which are much more efficient. Here is a calculator for ballscrews. For efficiency, a ballscrew is about 90% efficient.

My G0704 has the 570 oz-in steppers on all axes and has proven sufficient in X and Y but I would like a bit more on Z due to drilling forces. The ballscrew calculator says a 570 oz-in motor on a 0.196" (5mm) lead screw at 90% efficiency will give 1027 pounds of linear force. Using your 180 pound cutting force requirement there is a healthy margin. However, for a stepper design, the desired rapid speed may be the driver for motor sizing. This is because stepper torque drops off with increasing RPM and there will be a speed where the stepper will stall. For the 570 oz-in motors I can rapid reliably at about 150 IPM with double spring preload ball nuts. With single ball nuts that speed is closer to 200 IPM

You are correct on the calculator, I should have spend the extra couple minutes to find for specific for ball screws. I just changed the coefficient of friction 0.01.

Going from a single ball nut to a double you have to decrease your rapid speed by 25%? Is that typical?

Originally Posted by rmachinist

Here are the ball screws I just received

Getting exciting now! I would get some steel shim stock to put between the nuts but I guess the aluminum foil would work in a pinch. I have also read about people adding spring washers between them but you have to make sure your preload is greater than your cutting force.

Have you decided on the type of motors yet? I know I will never buy anything with just steppers again. I have had lots of fun with my shapeoko, including cutting the base of the 30MV https://photos.app.goo.gl/RBULexURfKht1DuTA, but I lost to many parts due to a missed step. If you can afford it I would go with some sort of closed loop system.

Re: started my cnc conversion for Precision Matthews PM-25MV Mill

Those grease nipples are a bit of a laugh too. How do you expect to get a grease gun on the Z nut when it's fitted???. You want some elbow fittings and 4mm tubing so you can put the grease nipples on the outside of the machine. Grease then flows from the nipple, through the tube, into the nut.
Unless you're doing a one shot oil system.[/QUOTE]

dazp1976,
i was thinking the same thing where did you buy your elbow fittings? you have a part numbers?

Re: started my cnc conversion for Precision Matthews PM-25MV Mill

You are correct on the calculator, I should have spend the extra couple minutes to find for specific for ball screws. I just changed the coefficient of friction 0.01.

Going from a single ball nut to a double you have to decrease your rapid speed by 25%? Is that typical?

Getting exciting now! I would get some steel shim stock to put between the nuts but I guess the aluminum foil would work in a pinch. I have also read about people adding spring washers between them but you have to make sure your preload is greater than your cutting force.

Have you decided on the type of motors yet? I know I will never buy anything with just steppers again. I have had lots of fun with my shapeoko, including cutting the base of the 30MV https://photos.app.goo.gl/RBULexURfKht1DuTA, but I lost to many parts due to a missed step. If you can afford it I would go with some sort of closed loop system.

Re: started my cnc conversion for Precision Matthews PM-25MV Mill

Going from a single ball nut to a double you have to decrease your rapid speed by 25%? Is that typical?

This is because my ballnut setup uses springs to force the ballnuts apart resulting in absolutely no backlash. The spring force is about 100-150 pounds driving them apart. On most double nut setups the ballnuts have shims between them to provide as little backlash as possible without binding, but because of screw lead errors the shimming cannot eliminate the backlash without binding the screw at some locations, so this method usually still has a little bit of backlash. Not much, but still a bit.

Have you decided on the type of motors yet? I know I will never buy anything with just steppers again. I have had lots of fun with my shapeoko, including cutting the base of the 30MV https://photos.app.goo.gl/RBULexURfKht1DuTA, but I lost to many parts due to a missed step. If you can afford it I would go with some sort of closed loop system.

I too would recommend some type of closed loop system if the budget allows. On my new build I am using closed loop steppers and they seem great for my expectations. I had originally planned to use servos because I wanted some type of following error detection, but I opted for the closed loop steppers as providing all the performance I really needed, easier setup than most servos, direct drive, and the following error detection. It was also a little cheaper too.

Having said that, I am still using and will continue to use open loop on other things because the closed loop stuff is quite a bit more expensive. On my G0704 I have not had any problems with losing steps as long as I don't push the machine too hard. It really comes down to a cost-benefit analysis that only you can do.

Re: started my cnc conversion for Precision Matthews PM-25MV Mill

Originally Posted by 109jb

This is because my ballnut setup uses springs to force the ballnuts apart resulting in absolutely no backlash. The spring force is about 100-150 pounds driving them apart. On most double nut setups the ballnuts have shims between them to provide as little backlash as possible without binding, but because of screw lead errors the shimming cannot eliminate the backlash without binding the screw at some locations, so this method usually still has a little bit of backlash. Not much, but still a bit..

Yep. I have a tiny bit of binding in places (minimal) after shimming. I've got down to 0.01mm on X and 0.025mm on Y. Near enough. I've gone a bit mad with 1100oz steppers which should cope. If anything the nuts will eat themselves before I have step problems

All this has made me strip the column again for the 4th time. Decided to rip out that crap DFU I just put in in have a go at getting my 2 single nut mashing design right. Took 0.13mm worth of shimming to get it to loosen off from where I had it. Seems to run smooth by hand now and looks like there is about 1 degree of free area. With any luck I'll be within a backlash of 0.03mm.

Re: started my cnc conversion for Precision Matthews PM-25MV Mill

Originally Posted by BrianPM

I didn't check my ballscrew lengths, only tested the ends to make sure they fit the bearings and preload nuts correctly. I'm just starting to machine the conversion parts now.
If Z or Y are too long, I figure I could cut/grind them down.
X- I can make my motor standoff a little taller.

I'm taking a shortcut with the motor standoffs to save a lot of time since I'm doing all the cutting manually.
I ordered these for the x and y mounts, they are a little tall, so I should just need to cut them down to the proper height. https://www.amazon.com/RATTMMOTOR-St...gateway&sr=8-6

I ordered a Nema34 mount as well, but its design is different (hollow) and I thought I would have to do significant modifications to get it to work, so I sent it back. Turning the standoffs on a lathe would be a lot easier in this case.

I just finished machinging my x-axis mounts this weekend, and took some measurements of the ball screw- the ball screw length looks to be dead on.
(Things are slow going, I've only had time to make chips late at night or earlty in the morning on weekends lately.)

Re: started my cnc conversion for Precision Matthews PM-25MV Mill

Originally Posted by BrianPM

I just finished machinging my x-axis mounts this weekend, and took some measurements of the ball screw- the ball screw length looks to be dead on.
(Things are slow going, I've only had time to make chips late at night or earlty in the morning on weekends lately.)

you have any pictures like to see what yours looks like compared to mine? i still need to buy a few more things I'm saving up for closed loop steppers, what type of fittings are you going using for greasing your ball screws?

I have a cheap small grease gun from walmart that just barely fits. This allowed me to grease everything with only removing the dust covers.although I will need an extension tube for the X. Hoping to just add 4" flexible grease hose.

I have a cheap small grease gun from walmart that just barely fits. This allowed me to grease everything with only removing the dust covers.although I will need an extension tube for the X. Hoping to just add 4" flexible grease hose.